One way to see the necessity of the two truths —instead of an elusive "the 'one' truth" (be it one of the two, or a third one)— is as follows:
• self-less, you can listen and understand the suffering of others (without 'your' individual biases and preferences interfering, gplus.wallez.name/64vMXWdHx2R),
• with a self, you can appropriate, make 'yours', take ownership of, take the responsibility of providing… an adequate response (gplus.wallez.name/LRLMG3PVvqS).

Seeing suffering without taking the responsibility to compassionately respond, is an extreme (gplus.wallez.name/Z38n35NGvyB). One can talk forever about how we're all connected and interdependent, but this doesn't solve hunger!
Engaging purely based on one's own perspective is another extreme. One can talk forever about being right, and might even have many reason to believe so, but this is still partial blindness… hence a good recipe to hit the wall (e.g. by causing resentment)!
Compassionately responding while taking into account the perspective (and induced suffering) of others is the Middle Way, beyond extremes. It is an engagement between, and beyond, the two truths: an engagement, not a mere ideal!

Not to confuse the Middle Way with a mere ideal —let alone an ideal solely based on the truth in ultimate terms— is important.
When one falls in the extreme of selfless response, one might be tempted to give everything they own for a cause (e.g. against poverty). Giving everything is meritorious, no doubt, but if one then "moves on", it become a past meritorious deed, not an engagement anymore. Some people do fall in this mistake, possibly spending the rest of this lifetime with pride and conceit associated to such meritorious deeds but with no further engagement… 'dead' deeds!

Buddhism makes clear that the "four immeasurables" (including compassion) are… immeasurables. If you do a good deed but then stop (possibly because you're not wise enough to see a way to continue), the deed becomes measurable, even if it was a compassionate response! The six pāramitās relate to numberless beings, innumerable gates, boundless ways… "Right effort" and the perfection of perseverance (and resilience) are related.
The truth in ultimate terms, when isolated from the truth in conventional terms, is not 'wisdom'. The engagement between the two truths, between the infinite wholesome qualities and the finite local practicalities, is where wisdom is, where the goal of holy life is.